U.S. Pat. Nos. 5,677,390, 5,686,538, 5,767,203, 5,852,131, 5,852,132, 5,916,978, 6,113,381, 6,207,765, 6,232,393, 6,232,408, 6,235,831, 6,235,844, 6,326,439, and 6,521,714 describe very desirable process technology for producing brominated styrenic polymers such as brominated polystyrene having the best known properties of any previously-known brominated styrenic polymer. In this connection, the terms “brominated styrenic polymer” and “brominated polystyrene” as used in the specification and in the claims hereof refer to a brominated polymer produced by bromination of a pre-existing styrenic polymer such as polystyrene or a copolymer of styrene and at least one other vinyl aromatic monomer, as distinguished from an oligomer or polymer produced by oligomerization or polymerization of one or more brominated styrenic monomers, the properties of the latter oligomers or polymers typically being considerably different from brominated polystyrene in a number of respects. Also as used herein, including the claims, the term “anionic styrenic polymer” or “anionic polystyrene”, terms commonly used by persons skilled in the art, denotes that the polymer referred to has been produced by use of an anionic polymerization initiator, such as a lithium alkyl. The terms “brominated styrenic polymer” and “brominated polystyrene” include and are generic to the terms “brominated anionic styrenic polymer” or “brominated anionic polystyrene”. These generic terms also include, for example, brominated styrenic polymers produced by bromination of styrenic polymers or polystyrene made by free radical polymerization.
As indicated in the above patents, the common method for recovering the brominated styrenic polymer from the liquid phase of the bromination reaction mixture involves deactivating the catalyst with an aqueous quenching medium, separating the brominated polymer-containing organic phase from the aqueous phase, stripping the organic phase of its solvent usually by feeding the organic phase into boiling water whereby the brominated polymer is precipitated, recovering the precipitated brominated polymer by a liquid-solids separation technique such as filtration or centrifugation, and drying the resultant finely-divided brominated styrenic polymer. It would be highly advantageous if a way could be found for recovering the brominated styrenic polymer in a more efficient and less costly manner.
When blending a brominated styrenic polymer with a substrate thermoplastic polymer to be flame retarded, it is desirable that the brominated styrenic polymer be in the form of pellets, granules, or pastilles. Unfortunately, a characteristic of brominated anionic styrenic polymers such as brominated anionic polystyrene, and to a lesser extent, brominated styrenic polymer made by free-radical polymerization as well, is a propensity to form substantial amounts of small particles and powders when attempts are made to pelletize the product. It appears that the pellets, unless bound together by an extraneous binding agent or the like, tend during formation, handling, and/or packaging to break apart and to revert to small particles and finely-divided powder, typically referred to as “fines”. Because of this characteristic, various conventional pelletizing or compacting procedures are unsuitable for producing brominated anionic styrenic polymers essentially free of fines and that during shipment and handling remain essentially free of fines. As can be readily appreciated, the existence or formation of fines in a product of this type is not only detrimental to the appearance of the pelletized product but in addition is undesired by the consumer.
In order to effectively use brominated styrenic polymers, especially brominated anionic styrenic polymers, as flame retardants in certain thermoplastic polymers, the use of binding agents or other extraneous materials to maintain the integrity of the flame retardant in pelletized form, is also deemed undesirable by some consumers. Thus, it is desirable to be able to produce unadulterated pelletized brominated styrenic polymers, especially brominated anionic styrenic polymers, that do not form undesirable amounts of fines during their preparation, packaging, shipment, and handling.
In the production of pelletized brominated styrenic polymers, especially brominated anionic styrenic polymers, it is important to improve the efficiency of the operation and to minimize operating costs. Recovery and recycle of fines avoids waste of product and minimizes waste disposal considerations. However the greater the amount of fines produced, recovered and recycled, the lower the production rate of pellets per quantity of polymer being subjected to pelletization. It would be highly advantageous, therefore, if a way could be found of preparing brominated styrenic polymers, especially unadulterated brominated anionic styrenic polymers, in pelletized form in a highly cost-efficient manner while avoiding formation of excessive amounts of fines, so that the production rate of high quality pellets is not materially impaired. In the case of brominated anionic styrenic polymers, it would be especially desirable if a way could be found of producing granules or pastilles of the polymers. This would enable the achievement of a number of important advantages in the process, including the overall process, the handling and storage of the product, and in the transportation of the product to, and the use of the product by, the end user.